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The gene product localizes to photoreceptor synaptic terminals in rods and mutations reduce synaptic transmission.
The gene product localizes to photoreceptor synaptic terminals in rods and mutations reduce synaptic transmission.

Night Blindness, Congenital Stationary, CSNB2B

Clinical Characteristics

Ocular Features

Night blindness is a feature of many pigmentary and other retinal disorders, most of which are progressive.  However, there is also a group of genetically heterogeneous disorders, with generally stable scotopic defects and without RPE changes, known as congenital stationary night blindness (CSNB).  At least 9 mutant genes are responsible with phenotypes so similar that genotyping is usually necessary to distinguish them.  All are caused by defective photoreceptor-to-bipolar cell signaling with common ERG findings of reduced or absent b-waves and generally normal a-waves.  The photopic ERG is usually abnormal to some degree as well and visual acuity may be subnormal.  In the pregenomic era, subtleties of ERG responses were frequently used in an attempt to distinguish different forms of CSNB.  Genotyping now enables classification with unprecedented precision.

In this disorder (CSNB2B) the b-wave responses are deficient (little or no scotopic response) and a-waves seem to be normal.  However, many if not most patients do not complain of night blindness.  Nystagmus, strabismus, and restriction of visual fields may be present.  Visual acuity is mildly to severely reduced.

Systemic Features

No systemic disease is associated with congenital stationary night blindness.

Genetics

CSNB2B, or type 2B, is one of four congenital nightblindness disorders with autosomal recessive inheritance.  It results from mutations in the CAPB4 (11q13.1) gene important in receptor to bipolar cell signaling.

Other autosomal recessive CSNB disorders are: CSNB1C (613216), CSNB (unclassified; OMIM number pending), and CSNB1B (257270).

Treatment Options

No treatment beyond correction of the refractive error is available but tinted lenses are sometimes used to enhance vision.

References

Khan AO, Alrashed M, Alkuraya FS. Clinical characterisation of the CABP4-related retinal phenotype. Br J Ophthalmol. 2012 Oct 25. [Epub ahead of print].

PubMed ID: 
23099293

Berger W, Kloeckener-Gruissem B, Neidhardt J. The molecular basis of human retinal and vitreoretinal diseases. Prog Retin Eye Res. 2010 Sep;29(5):335-75.

PubMed ID: 
20362068

Zeitz C, Kloeckener-Gruissem B, Forster U, Kohl S, Magyar I, Wissinger B, M√°ty√°s G, Borruat FX, Schorderet DF, Zrenner E, Munier FL, Berger W. Mutations in CABP4, the gene encoding the Ca2+-binding protein 4, cause autosomal recessive night blindness. Am J Hum Genet. 2006 Oct;79(4):657-67.

PubMed ID: 
16960802